The invention relates to a filter for medical fluids, in particular fluids intravenously administered to the patient.
It must be ensured that medical fluids administered to a patient do not contain air or other gases. To remove the air from the medical fluid filters with a hydrophilic and a hydrophobic membrane are used. When hydrophilic membranes have been wetted with the fluid they are fluid-permeable and air-impermeable up to a limiting pressure. Hydrophobic membranes are always air-permeable and fluid-impermeable.
Such a filter is, for example, described in EP 0 302 722 B1. The filter comprises a flat housing divided by the hydrophilic membrane, which in the case of the present filter extends over the overall filter cross-section, into an inlet and an outlet chamber. The complete inner wall of the two chambers is formed by the hydrophilic filter membrane. For venting the inlet chamber two vent holes covered with a hydrophobic membrane are arranged opposite the hydrophilic membrane. In vertically arranged filters the two vent holes are provided in an upper and a lower section. To ensure reliable venting of the inlet chamber care must be taken that the filter is always arranged in such a way that the vent holes point upwards.
U.S. Pat. No. 4,294,594 discloses a filter comprising an inlet chamber and an outlet chamber in a housing. Between the two chambers an intermediate chamber separated from the inlet chamber by a deep-bed filter is arranged. The intermediate chamber is separated from the outlet chamber by a hydrophobic filter membrane. At the upper end of both the inlet chamber and the intermediate chamber a vent hole with a hydrophobic membrane is provided. These vent holes are located at that end of the housing which forms the upper end when the filter is properly arranged. However, proper arrangement of the filter cannot always be ensured, in particular when the filter is attached to the patient""s lower arm since the lower arm can, of course, assume different positions. If air penetrates the infusion system, for example when the filter is upside-down, the air does not leave the housing when the filter has been wetted.
It is the object of the present invention to create a filter for medical fluids with reliable venting.
The filter according to the present invention has at least two vent holes, both covered with a hydrophobic membrane, one of which is arranged in a front wall opposite the hydrophilic membrane and the other in the rear wall. Thus the vent holes are provided in filter walls arranged opposite each other and at opposite ends of the housing. Since at least two vent holes point in opposite directions, there is always one vent hole pointing upwards irrespective of the horizontal arrangement of the filter. Air or other gases can easily escape through the vent hole pointing upwards, even when the filter is only slightly inclined. It is thus ensured that no air bubbles accumulate in the inlet chamber, which would not escape or escape only very slowly owing to the filter position and would thus affect the flow rate.
In a particularly preferred embodiment of the filter according to the invention at least one hydrophobic membrane extends over almost the overall filter width at the level of the corresponding vent hole, with the filter width possibly varying in the longitudinal extension of the filter. In this way air bubbles are collected by this hydrophobic membrane over the overall filter width at that location and discharged through the vent hole. The hydrophobic membrane extending over the overall filter width can be connected with a plurality of vent holes. Further, the corresponding vent hole can also extend over almost the overall filter width and possibly comprise crossbars to fix the hydrophobic membrane.
In the vertically arranged filter one of the vent holes is preferably arranged in the upper section and another vent hole in the lower section of the inlet chamber. When the filter is used in a vertical infusion tube as is frequently the case, it is thus ensured that during administration of the medical fluid to the patient venting of the inlet chamber is substantially effected through the upper vent hole. When the filter is filled with fluid, venting is essentially effected through the lower vent hole. This allows uniform flow of the fluid into the inlet chamber.
The hole which forms the lower vent hole in the present configuration is preferably the vent hole provided in the rear wall. Thus no hydrophilic membrane is arranged opposite the lower vent hole. This prevents air bubbles from accumulating at the hydrophilic membrane local to the lower vent hole and affecting uniform venting of the inlet chamber in particular when the filter is being filled.